Required section · Section 1 of 6

A number without a timeline

A vancomycin result of 11.8 µg/mL is available at 17:45, but the trough specimen was collected at 16:45, 15 minutes before dose 6 begins at 17:00. A post-distribution specimen collected at 07:30 after dose 5 measured 22.1 µg/mL. The laboratory question is whether the documented specimen times support a steady-state model, not whether a dose should change.

Therapeutic drug monitoring, or TDM, is useful when a drug has a narrow therapeutic index, variable pharmacokinetics, or uncertainty about adherence, toxicity, or efficacy. A concentration only gains meaning when dose, route, exact last-dose time, draw time, specimen type, and clinical question travel with it. The laboratory releases the result and clarifies evidence limits; it does not prescribe a dose.

A level drawn from the line that carried the drug can be falsely elevated from catheter dead space. A level with no timing is a random level and may not fit a target developed for a peak, trough, or area-under-the-curve approach. Record the missing facts before escalating an unexpected result.

Treat a concentration and its collection history as one result.

Mock TDM requisition showing drug and method, dose and route, infusion end, collection time, specimen source, and clinical question. — Figure 1Fields needed to make a TDM result interpretable.

Knowledge checks

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Knowledge check 1

Which missing fact most directly prevents interpreting a reported drug level as a trough?

Choose one option.

Exact collection time relative to the next dose

Whether the result was highlighted in the LIS

Name of the collecting staff member

Section status

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